This invention relates to an arrangement for extending the range of radio coverage in a TDMA (Time division Multiple Access) system. The invention will be described with reference to a digital GSM cellular radio system using TDMA, but is applicable to certain classes of TDMA systems generally.
Radio TDMA systems are particularly useful when a central transceiver deals with a plurality of remote transceivers. However, the distance between the remote transceivers and the central transceiver introduces propagation delay which varies in direct proportion to the distances the remote transceivers (mobiles) are located from the central transceiver (base station).
To prevent adjacent transmissions from mobiles overlapping in time due to propagation delay it is necessary to control the times allocated for transmission (timeslots) from the mobiles. To maximise information throughput it is well known that each individual remote transceiver can be instructed to advance the starting time of its transmission in order to compensate for the propagation delay and to maximise spectrum efficiency.
Cellular radio telephone systems divide a coverage area into a plurality of contiguous or overlapping cells each served by a base station. In urban areas, the coverage of each cell may be only a few kilometers in diameter, cell size being reduced as the communication traffic density increases.
As the traffic density decreases the size of the cells can be increased provided radio coverage remains adequate for successful communication. The protocol adopted by GSM permits mobiles to advance their timing by a maximum of 63 communication bits. This is equivalent to approximately 35 kilometers. Once the mobile exceeds this distance it cannot be instructed to advance its timing any further, Consequently the base station receiver will eventually be unable to correlate the received mobile transmission once the optimum timing advance becomes too great. Transmissions from beyond 35 Km will then overlap into the following timeslot at the base station receiver.
Radio propagation conditions are favourable for extending the range of a GSM communication system beyond 35 kilometers when the base station is located atop a central highpoint (e.g., 300 meters) and the surrounding intended coverage area is largely flat. Under these circumstances it is far more economic to extend the coverage area of the base station beyond 35 kilometers than to install additional sites and base stations.